Waterjet cutting apparatus has been used for some years to cut a variety of materials such as steel, aluminium, glass, marble, plastics, rubber, cork and wood. The work piece is placed over a shallow tank of water and a cutting head expelling a cutting jet is accurately displaced across the work piece to complete the desired cut. The cutting action is carried out by the combination of a very high pressure jet (up to 90,000 psi) of water entrained with fine particles of abrasive material, usually sand, that causes the cutting action. The water and sand that exit the cutting head are collected beneath the work piece in the tank.
It is in the industry associated with waterjet cutting that the expression “ultra high pressure” (UHP) waterjets are used to define a process where water is pressurised above 50000 psi and then used as a cutting tool. The high pressure water is forced through a very small hole which is typically between 0.1 mm and 0.5 mm in diameter in a jewel which is often ruby, sapphire or diamond.
Typically, two types of pumps are used to create the high pressure water, namely:                a) intensifier pumps; and        b) direct drive crank pumps.        
Intensifier pumps are usually hydraulic pumps which can pressurise oil up to approximately 3000 psi. The oil is then forced into a cylinder which has a large piston attached to a smaller piston which is 1/20 the area of the hydraulic piston. The secondary piston is positioned in a cylinder which is filled with water. As the hydraulic piston is forced back and forth it forces the water piston to reciprocate creating a pressure some twenty times that of the hydraulic system. Although these systems are fairly reliably, they are inefficient due to the need to drive the hydraulic system. Typically, these pumps run at about 55% efficiency.
A more efficient pump is the direct drive crank shaft pump where a motor is coupled directly to a crank shaft. The crank shaft rotates whilst driving a number of small pistons, usually three, to reciprocate in cylinders thus pressurising the water. These pumps are fairly efficient, typically above 80%, when they are utilising the water being pressurised but they cannot store and hold pressure which means that when the waterjet apparatus is not actually cutting, the pressurised water is expelled from a release valve which means that the pumps use a similar amount of power whether sitting idle or in a cutting operation. These pumps are not as reliable as the intensifier pumps due to the high piston speed and the number of strokes required to make the same volume of ultra high pressure water.
It is the limitations of the pumps described above that have brought about the present invention.